Molecular transparence and contrast with the STM: a theoretical comparison of carbon monoxide and ethylene

Since carbon monoxide and ethylene have different chemisorption modes on a platinum (111) surface (carbon monoxide is perpendicular to the surface and ethylene is parallel), one would expect a large difference in the transmission of tunnelling electrons for these adsorbates. However, the features obtained with a high-resolution scanning tunelling microscope (STM) only show subtle qualitative differences. For ethylene, a di-sigma chemisorption mode (or bridge) has been considered as suggested from experiments, together with a less stable pi-mode model. Chemisorbed ethylene appears in our STM simulation images as a circular bump for both structures with depressions in the carbon-carbon direction only for the di-sigma mode. Because of these depressions, the bump corresponding to the di-sigma mode appears to be slightly elliptical but in the direction perpendicular to the carbon-carbon bond. However, the main difference between the molecules concerns the amplitude of the patterns in the topographic mode. For the bridge site, it is about three times higher for ethylene than for carbon monoxide. The difference is less striking for the top site. In this paper, we explain both the shape similarity of STM images and the important difference in the corrugations with the help of a theoretical simulation model and its current analysis tool. The contrast difference arises from interference effects between molecular orbital contributions in the tunnelling current. (C) 1998 Elsevier Science B.V. All rights reserved.